The invention consists of a self-threading screw which includes a (basically) conical point set at the end of its shank opposite that where the head is housed, and a sunken zone set between the end of the screw shank and the conical point; in addition, an apparatus to produce such a self-threading screw is disclosed, in which at least the thread of the self-threading screw, the conical point and the sunken zone are simultaneously produced by rolling.
Self-threading screws (i.e. screws suitable to produce, on the wall of the holes into which they are inserted, a thread of complementary profile to that of the thread on the screw shank) have been known for a long time, widely available on the market and much used in the technique to join two parts made of metallic material.
Screws have been put on the market, the shank of which has, at its free end, four or (preferably) six prismatically shaped xe2x80x9cmouldersxe2x80x9d set at regular intervals along the external circumference of the free end of the screw shank: such moulders are suitable to roll the material making up the body to be threaded thus obtaining, along the wall of the hole (without requiring any additional mechanical operations), a highly resistant complete screw thread, whose threads have intact fibres and are slightly hardened by the pressure generated by the same moulding.
The known type of self-threading screws, produced by rolling and including four or (preferably) six prismatically shaped moulders are very convenient and advantageous to thread the walls of a hole, but are not fit to be used to assemble two plates with holes, whose holes are rarely exactly in line with each other.
In fact, the free end of the self-threading screws currently available have a truncated form (or, in any case, unpointed) which makes it extremely difficult (if not practically impossible) to insert such screws into the hole present in the plates to be assembled if such holes are not perfectly aligned.
This problem, widespread in the industry of light metal structural work, had still not been solved in an economic way since it was not possible, until now, to produce by rolling a screw having a pointed end without having to resort to additional mechanical operations, thus resulting in a high screw cost which puts the same screw outside the market: in fact it has not been possible until now to thread the screw shank by rolling and simultaneously form the point since (with the apparatuses used so far) the stress applied to the screw shank to form the point tends to expel the screw itself from the thread chasers used to thread the screw shank and to make any moulders on it, resulting in serious damage to the screw thread.
Experimental research and tests carried out by the applicant have found a solution for this xe2x80x9ctechnical inconveniencexe2x80x9d, producing a self-threading screw by rolling, which includes a (basically) conical point by using an apparatus, derived from that normally used to produce a self-threading screw of known type, which allows the simultaneous production of at least the screw thread of the self-threading screw and of its conical point, without risking damaging the screw thread itself.
The subject of the present invention is a self-threading screw which includes a (basically) conical point set at the end of its shank opposite that where the head is housed, and a sunken zone set between the end of the screw shank and the conical point.
An additional subject of the present invention is an apparatus suitable to produce a self-threading screw having a conical point and a sunken zone, which includes an additional pair of thread chasers (set under those which produce the thread and any moulders of the self-threading screw) suitable to produce the conical point and the sunken zone: the thread (and any moulders) of the self-threading screw, the conical point and the sunken zone are simultaneously produced by rolling.